It is widely practiced in the electronics industry to form electronic circuits by printing and firing thick film pastes. Such pastes are usually dispersions of finely divided particles of conductive metals and inorganic binder (glass frit) in an organic medium comprising a solid organic polymer dissolved in a volatilizable solvent. When circuits prepared in this manner are located on the outside of the electronic device, it is desirable to protect them from abrasion during handling and from environmental hazards by covering them with a coating of low melting glass. Such circuits may be either conductive or resistive in electrical functionality. Conventionally, the glass used for such applications has been an amorphous lead borate-type glass.
However, a recent trend toward the use of higher density circuitry has resulted in a demand for finer lines and spaces between the conductor lines. This, in turn, has exacerbated the problem of the generation of mechanical stresses in such structures because of the differences in the coefficients of thermal expansion (TCE) between the substrate and the printed and fired resistor or conductor. These higher stresses are exhibited by the appearance of microcracks which change the electrical properties of the functional layer. Thus, the resistance of a fired thick film resistor would be changed substantially by the presence of very small cracks in the printer and fired resistor layer. For this reason, there has been a substantial need for a way to suppress the formation of these mechanical stresses and thus to avoid unwanted changes in the resistance values of printed electronic circuits.